1. Field of the Invention
This invention relates generally to solid state controllers having utility in industrial processes and adapted to respond to a process signal for operating a motorized valve. The invention is concerned with that type of controller that is adapted to provide so-called proportional speed floating control. The controller according to the present invention is particularly concerned with a solid state electronic power relay that is operative in response to a d-c time-proportioned signal transmitted over a low voltage and low current two-wire transmission line to regulate the speed and direction of an electric motor valve actuator and thereby a final control element to restore and maintain an industrial process at a desired value. The controller is particularly suitable for flow rate, pressure, viscosity, liquid level, and other industrial processes having short time constants.
2. Description of the Prior Art
Proportional speed floating control is a type of control action in which the direction and rate of adjustment of the final control element, such as a valve, damper, etc., is proportional to the direction and extent of deviation, from a desired set point, of the process variable that is being controlled. The final control element is said to float since the position of adjustment thereof can be anywhere within the operating range when the deviation is zero.
Proportional speed floating controllers accept voltage or current input signals from primary sensors responsive to process variables to provide, for example, rate of flow control, by means of motor driven valves, of water, sewage, sludge, slurry, and other process fluids. Such controllers compare the signal from the flow or other process variable transmitter with a set point signal representing a desired flow rate. Such set point signal may be either locally or remotely generated. When a difference appears between the actual and the desired process variable value, the controller provides output signals that regulate the supply of energizing current to a reversible electrical motor for rotation at a speed and in a direction to operate a control valve as required to restore the process variable to the desired value. The basic control mode is proportional velocity. That is to say, the output signals of the controller are speed signals that are proportional to deviation and that are integrated by the electrical motor as the latter drives to the correct valve position.
Controllers of this type may be employed to directly control the operation of a light-duty motorized valve having, for example, a rating of one ampere or less. In modern industrial process control systems, however, heavy-duty motorized valves having much higher power requirements are used. When such controllers are employed to operate such heavy-duty motorized valves, a power relay must be employed to control the required heavy current and or high voltage to the motorized valve. Such power relays in some applications are located adjacent the motorized valve with the controller output signals being transmitted to the power relay over a three-wire, relatively high voltage, transmission line. When so located energizing current is supplied to the power relay from a locally available source.
Solid state power relays utilizing semi-conductor switches of the type normally referred to as an SCR, triac or thyristor have been proposed in the prior art because of their heavy current and high voltage handling capabilities. The prior art power relays, however, have had certain practical drawbacks that have affected their reliability and suitability for certain applications, which drawbacks have been evident, particularly, in connection with the use of such power relays in energizing heavy-duty single-phase and three-phase alternating current reversible electrical motors. The high rate of current change with time at the moment of firing such semiconductor switches has been of utmost significance in connection with the reliable opration of such power relays, both due to the failure of the semiconductor switches themselves and also the circuitry to which the switches are connected.